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Numerical Simulations of Penetration and Overshoot in the Sun
Tamara M. Rogers
Astronomy and Astrophysics Department, University of California, Santa Cruz, CA 95064
Gary A. Glatzmaier
Earth Sciences Department, University of California, Santa Cruz, CA 95064
C.A. Jones
Department of Applied Math, University of Leeds, Leeds LS2 9JT, UK
ABSTRACT
We present numerical simulations of convective overshoot in a two-
dimensional model of the solar equatorial plane. The model equations are solved
in the anelastic approximation with enhanced thermal conductivity and viscosity
for numerical stability. The simulated domain extends from 0.001 R to 0.93 R,
spanning both convective and radiative regions. We show that convective pen-
etration leads to a slightly extended, mildly subadiabatic temperature gradient
beneath the convection zone, spanning approximately 0.05 Hp, below which there
is a rapid transition to a strongly subadiabatic region. A slightly higher tempera-
ture is maintained in the overshoot region by adiabatic heating from overshooting
plumes. This enhanced temperature may partially account for the sound speed
discrepancy between the standard solar model and helioseismology. Simulations
conducted with tracer particles suggest that a fully mixed region exists down to
at least 0.684 R.
Subject headings: convection: overshoot,mixing
1. Introduction
One of the main unsolved problems in all of stellar evolution theory is the treatment
of convective-radiative boundaries. In the Sun, understanding convective overshoot is cru-
cial in determining properties of the solar tachocline and the solar dynamo. Helioseismic
– 2 –
observations have shown that the differential rotation observed at the solar surface persists
throughout the convection zone (Thompson et al. 1996). More surprisingly, the p-mode
splittings showed that over a very thin radial extent the rotation profile changes from differ-
ential in the convection zone, to solid body in the radiative interior. The unresolved radius
over which this transition occurs has been named the tachocline (Spiegel & Zahn 1992).
Whether this region is quiescent and mostly devoid of overshooting motions or is violent and
constantly bombarded by plumes is an unanswered question, the solution to which may help
constrain theoretical models for the tachocline.
Understanding the tachocline is not only important for comprehending the internal
rotation of the Sun; it is crucial for understanding the dynamo process. In classic mean field
theory, the dynamo process is explained in two steps: poloidal field is sheared into toroidal
field by differential rotation (the Ω effect), toroidal field then buoyantly rises and, because
of Coriolis forces is twisted back into a poloidal field (the α effect). These processes were
initially postulated to occur in the convection zone. However, it became clear some time ago
(Parker 1975) that magnetic field would become buoyant and be quickly shredded within
the turbulent convection zone. During the ensuing years it was proposed that the storage
and amplification of field could occur in the stably-stratified overshoot region (Spiegel &
Weiss 1980). The tachocline, with its strong differential rotation would provide the ideal
site for storage and amplification of the field. To explain how the αΩ dynamo would work
in this scenario Parker (1993) proposed the interface dynamo, which places the Ω effect in
the stable region beneath the convection zone, while keeping the alpha effect in the bulk
of the convection zone. If the interface dynamo is to work, the overshoot region must play
two crucial roles in the dynamo cycle: (1) the stable stratification allows field, which is
transported into the region by overshooting plumes (Tobias et al. 1998), to be stored there
on the solar cycle timescale and (2) the strong shear in this region must convert poloidal field
into toroidal field. Understanding the precise nature of this overshoot region, the amplitude
of the subadiabaticity and its depth, is crucial for understanding the efficiency with which
field can be pumped into the overshoot region, the magnitude of the field capable of being
stored there and the timescale on which it could be stored.
The problem of overshoot is not confined to the Sun. Most stages of stellar evolution
are affected. The Lithium depletion in some main sequence stars may be explained by
convective overshoot. In more massive stars with convective cores, overshoot can lead to
increased central hydrogen abundance, and therefore longer main sequence lifetimes, which
in turn affects isochrone fitting and age predictions for stellar clusters. Dredge up during the
Asymptotic Giant Branch (AGB) phase can lead to the surface enrichment of material from
the core and to the presence of 13C necessary for s-process nucleosynthesis (Herwig 2000).
Convective overshoot can affect the energy production and nucleosynthesis of classical novae
– 3 –
by enriching the accreted layer with underlying white dwarf material (Woosley 1986).
Clearly, overshoot is an important physical process which must be explained if stellar
evolution is to be better understood. Many analytic and numerical theories have been pro-
posed, but no clear consensus has been reached. Early analytic models (Shaviv & Salpeter
1973, van Ballegoojen 1982, Schmitt, Rosner & Bohn 1984) predicted extended adiabatic
regions and early numerical experiments concurred (Hurlburt et al. 1994). More recent
analytic results (Zahn 1991, Rieutord & Zahn 1995, Rempel 2004) model the penetration
using plume dynamics and agree that the extent and nature of the overshoot depends crit-
ically on the filling factor of the plumes at the base of the convection zone as well as the
flux. It also appears that the inclusion of upflow-downflow interaction, and the prescription
for it, affects the penetration depth (Rieutord & Zahn 1995, Rempel 2004). Numerous nu-
merical experiments have also been conducted. Two-dimensional models (Massaguer et al.
1984, Hurlburt et al. 1986, Hurlburt et al. 1994, Rogers & Glatzmaier 2005a) have been
conducted to determine the nature of convective overshoot as well as dependencies of the
penetration depth on other physical variables such as stability of the stable region and the
input flux. Three-dimensional simulations have also been conducted (Chan & Sofia 1989,
Singh et al. 1998b, Saikia et al. 2000, Brummell et al. 2002). While the earlier three di-
mensional models recover the scaling relationship between penetration depth and flux (rms
velocity) determined analytically and in two dimensional numerical experiments, the more
recent numerical simulations find that this scaling law breaks down at higher levels of the
stability of the underlying radiative region (Brummell et al. 2002).
All analytic predictions make some crude assumptions, while all numerical simulations
employ far from realistic solar parameters. The numerical simulations presented in this work
still can not reach the level of turbulence that surely exists in the Sun. However, in this work
we have taken a few steps forward by using a realistic thermal profile, including rotation,
employing a more realistic geometry and including most of the convective and radiative
regions. In section 2 we describe our numerical model and equations; in section 3 we discuss
some of the features of basic convective penetration. In section 4 we present estimates for
the depth of the penetrative convection before our reference state model is evolved. Section
5 details the evolution of the mean thermal profile and section 6 compares these results with
previous analytic and numerical models. In section 7 we discuss mixing of species as modeled
by tracer particles.
– 4 –
2. Numerical Model
2.1. Equations
The numerical technique used here is identical to that in Rogers & Glatzmaier (2005b).
We solve the Navier Stokes equations in the anelastic approximation (Gough 1969) for per-
turbations about a mean thermodynamic reference state. The equations are solved in 2D
cylindrical geometry, with the computational domain extending from 0.001 R to 0.93 R,
representing the equatorial plane of the Sun. The radially (r) dependent reference state is
taken from a standard 1D solar model (Christensen-Dalsgaard private communication). The
radiative region spans from 0.001R to 0.718R, while the convection region occupies the
region from 0.718R to 0.90R and for numerical ease an additional stable region is included
from 0.90R to 0.93R. Excluding 0.1 % of the solar radius in the core should have little
effect, as gravity waves reflect when their frequency approaches the Brunt-Vaisala frequency,
which vanishes at the core.1
For numerical simplicity we solve the curl of the momentum equation, the vorticity
equation:
∂ω
∂t+ (v · ∇)ω = (2Ω + ω)hρvr − g
Tr
∂T
∂θ− 1
ρTr
∂T
∂r
∂p
∂θ+ ν∇2ω (1)
where, ω = ∇ × v is the vorticity in the z direction and v is the velocity, comprised of
a radial component, vr, and a longitudinal component, vθ. T is the temperature, ρ is the
density, p is the pressure, and g represents gravity. Overbars denote prescribed reference
state variables taken from a 21st order polynomial fit to the 1D solar model. These values are
functions of radius and slow functions of time when the reference state is allowed to evolve
(see section 2.2). Variables without overbars are the time dependent perturbations which
are solved for relative to the reference state. Derivatives of the reference state values are
calculated from analytic derivatives of the polynomial expansions. The viscous diffusivity, ν,
is radially dependent and defined such that the dynamic viscosity, νρ is constant (initially).
The rotation rate, Ω, is set equal to the mean solar rotation rate, 2.6×10−6 rad/s.
The energy equation is solved as a temperature equation:
∂T
∂t+ (v · ∇)T = −vr(
∂T
∂r− (γ − 1)Thρ)+
1However, we do not fully understand the properties of nonlinear waves under internal reflection. It ispossible that internal reflection is physically distinct from reflection off of a hard boundary.
– 5 –
(γ − 1)Thρvr + γκ[∇2T + (hρ + hκ)∂T
∂r]+
γκ[∇2T + (hρ + hκ)∂T
∂r] +
Q
cv(2)
In Equation (2), κ is the (radially dependent) thermal diffusivity, (γ − 1) = (dlnT/dlnρ)ad,
hρ = dlnρ/dr and hκ = dlnκ/dr. These last two, hρ and hκ, represent inverse scale heights.
Q represents physics included in the 1D standard solar model, but not included here, which
maintains the initial reference state temperature gradient. In the convection zone, Q is the
divergence of the mixing length flux which, together with the second to last term in (2)
accounts for the convergence of the total reference state flux through the system. Initially,
this sum is set to zero, so that the initial reference state temperature is time independent.
In this model, similar to Rogers & Glatzmaier 2005b, we use the temperature as our
working thermodynamic variable, rather than entropy. We do this to avoid the inward heat
flux that would accompany a positive entropy gradient and large turbulent diffusivity in the
stable, radiative interior.
We calculate the pressure term in (1) by solving the longitudinal component of the
momentum equation:
1
ρr
∂p
∂θ= −∂vθ
∂t− (v · ∇v)θ + ν[(∇2v)θ − hρ
3r
∂vr
∂θ] (3)
These equations are supplemented by the continuity equation in the anelastic approximation:
∇ · ρv = 0 (4)
The convective and radiative regions are set up by taking the subadiabaticity defined
as:
∆∇T = (γ − 1)hρT − ∂T
∂r≈ ((
∂T
∂r)ad − ∂T
∂r) (5)
from the 1D model and specifying the superadiabaticity in the convection zone as the con-
stant value 10−7 K/cm. The thermal diffusivity, κ is given by the solar model, multiplied by
a constant factor, κm for numerical stability:
κ = κm16σT
3
3ρ2kcp
(6)
where σ is the Stefan-Boltzman constant, k is the opacity (taken from the 1D solar model)
and cp is the specific heat at constant pressure. The multiplying factor, κm, is 105 and
therefore, the convective heat flux is 105 larger than the solar value. This produces the
– 6 –
proper radial profile of the radiative diffusivity, albeit increased by a large factor for numerical
stability; this is a “turbulent” diffusivity.
Since both ν and κ vary with height, the relevant control parameters, such as Pr (Prandtl
number = ν/κ), Ra (Rayleigh number =g∆∇TD4/νκT , where D represents the convection
zone depth) and Ek (Ekman number =ν/(2ΩD2)) vary with height. The Pr varies from
10−2BCZ (at base of the convection zone) to 0.7TCZ (at top of convection zone) and is 10−3 near
the core. The Ra varies from ≈ 108BCZto ≈ 107
TCZ and the Ekman number (Ek=ν/2ΩD2)
varies from 10−4BCZ to 10−2
TCZ .
These equations are solved using a Fourier spectral transform in the longitudinal (θ)
direction and a finite difference scheme on a non-uniform grid in the radial (r) direction.
Time advancing is done using the explicit Adams-Bashforth method for the nonlinear terms
and an implicit Crank-Nicolson scheme for the linear terms. The domain resolution is 2048
x 1500, with 620 radial levels dedicated to the radiative region. The radial resolution in
the overshoot region is 170km. The boundary conditions on the velocity are stress-free and
impermeable, while the temperature boundary condition is specified as constant temperature
gradient at the top and constant temperature at the bottom.
2.2. Updating the Reference State
After the model had run ≈ 1 year (which requires nearly 4 million timesteps), we allow
the mean reference state to evolve in response to the convection. The procedure is as follows:
(1) Half of the mean temperature, density and pressure perturbations (T (m = 0 , r),
ρ(m = 0 , r) and p(m = 0 , r)) are added to the reference state values T , ρ, p forming the
new reference state:
T new = T old +1
2T (m = 0 , r) (7)
ρnew = ρold +1
2ρ(m = 0 , r) (8)
P new = P old +1
2P(m = 0 , r) (9)
Here, m is the longitudinal wavenumber and m=0 represents the axisymmetric (mean) per-
turbation. The remainder of the mean perturbation is maintained as a perturbation so that
the model can adjust gradually.
(2) Using the new values for density and temperature, a new opacity is calculated using
Kramers Law
k(r) = C(r)ρnew(r)T new(r)−3.5 (10)
– 7 –
where C(r) is obtained by matching the original opacity profile, taken from the 1D solar
model, to a Kramer’s Law opacity, i.e. C(r)ρold(r)Told(r)−3.5 = k(r)1Dsolarmodel.
(3) The thermal diffusivity κ is recalculated with the new opacity, temperature and
density via equation (6), the multiplying factor κm is maintained.
(4) The last two terms in (2) are updated, holding Q constant.
Initially, this procedure is done at regular intervals. After the initial relatively large
changes, this procedure is done only if the mean thermodynamic perturbations become
larger than 1% of the reference state values.
3. Convective penetration and Gravity Wave generation
Turbulent convection is dominated by plumes, which form near the top of the convection
zone and descend through the bulk of the convection zone. These plumes tilt and sway and
often merge with neighboring plumes during their descent. Within the convection zone the
kinetic energy spectra varies between m−2 and m−4 depending on the radius at which it is
measured and the Ekman number. The plumes terminate their descent upon encountering
the stiff underlying stable region (Figure 1). In this region a downwelling plume adiabatically
heats relative to the subadiabatic background (hence the white “spots” seen in Figure 1 at
the base of the convection zone) and is then rapidly decelerated by buoyancy. The depth over
which the plume is buoyantly stopped depends sensitively on the stiffness of the underlying
radiative region (Hurlburt et al. 1994, Brummell et al. 2002, Rogers & Glatzmaier 2005a).
When the plumes impinge on the underlying radiative region they generate a spectrum
of gravity waves, ranging in frequency from 1µHz to 300µHz (Rogers & Glatzmaier 2005b).
Higher frequency waves set up standing modes which may ultimately be detected by he-
lioseismology. The lower frequency waves are radiatively dissipated, thereby sharing their
angular momentum with the mean flow.
4. Convective Overshoot
There are several ways to define the depth of convective overshoot. In this section
we consider only the depth to which subadiabatic overshooting motions can extend into the
stable radiative region. In the next section we consider whether these motions can extend the
adiabatic region beyond that prescribed by the reference state model. As we are concerned
with the cessation of large amplitude motion, we consider the amplitudes of kinetic energy
– 8 –
density ρ(v2r +v2
θ), vertical kinetic energy flux vr(kinetic energy density) and convective heat
flux (ρTvrcp) as a means for determining the depth of convective overshoot. The time and
horizontally averaged kinetic energy density as a function of radius is shown in Figure 2a.
The kinetic energy density drops rapidly moving into the radiative region because of the stiff
underlying stable region. The peak energy density is ≈ 5 × 108ergs/cm3, but drops to 104
ergs/cm3 at just one Hp (pressure scale height) below the convection zone 2. If we define
the depth of overshoot to be the distance between the base of the convection zone and the
height at which the kinetic energy density reaches 1% of its peak value, then that depth is
5.3×108cm, or 0.09 Hp (Hp is the pressure scale height at the base of the convection zone,
5.8 × 109cm). If instead we use 5% of the peak, the depth becomes 0.06 Hp.
An alternative, more physical way of defining the depth of convective overshoot is based
on where the kinetic energy flux changes sign. The average vertical kinetic energy flux3
(Figure 2b) is negative (downward) in the bulk of the convection zone due to descending
plumes. In this model, because of the stiffness of the underlying radiative region, the flux
changes sign well within the convective region and is positive at the base of the convection
zone. In this region descending plumes rebound, causing a net upward flux. The extent of
the region with positive vertical kinetic energy flux could be considered an overshoot depth.
This yields a smaller depth of 0.02 Hp.
Another physical measure of overshoot can be found if one considers the correlation
between temperature perturbation and vertical velocity i.e., the convective heat flux. Typ-
ically, a positive temperature perturbation (hot fluid) would give rise to a positive vertical
velocity. However, in the overshoot region, positive temperature perturbations are associ-
ated with negative vertical velocities. Therefore, in the overshoot region the quantity Tvr is
negative, as seen in Figure 2c, leading to a similar measure of the overshoot depth, 0.03Hp.
The reduction in kinetic energy density from peak is 20% for 0.03Hp penetration depth and
28% for 0.02Hp.
5. Evolving the Reference State
In order to study how the overshooting motions discussed above affect the mean ther-
mal state of the system, we allow the model to evolve in response to those motions. We can
2Note that the kinetic energy density presented here is larger than one might expect in the Sun. This isdue to increased velocities driven by a larger than solar thermal diffusivity.
3Plotted in Figure 2b is the kinetic energy flux divided by the reference state diffusive heat flux,κρcpdT/dr.
– 9 –
then determine if we find an extended adiabatic region or just subadiabatic overshoot. An
extended adiabatic region is generally referred to as “penetration depth” and distinguished
physically and colloquially from overshoot, or just subadiabatic overshoot. After the model
has run nearly one year the reference state thermodynamic variables are updated. The
horizontal average of the temperature perturbation (i.e. the m=0 mode of the Fourier ex-
pansion) is added to the reference state temperature, as described in section 2.2. Similarly,
the horizontally averaged density and pressure perturbations are added to their reference
state values. The newly formed density and temperature are used to define a new opacity
using Kramers Law (10). Using the new opacity and thermodynamic variables a new thermal
diffusivity is obtained using (6). A new reference state is thus formed. Initially, this proce-
dure is done periodically. Later, the reference state is changed only when the temperature
perturbation reaches 1% of the mean temperature at any location.
At any given instant the temperature perturbations are a small fraction of the mean tem-
perature; however, the continual evolution of the temperature, density and opacity produce
large changes in the mean thermal profile (Figure 3). In Figure 3a we show the super-
/subadiabaticity, as defined in (5), for the initial model (solid line) and our evolved model
(dotted line); in Figure 3b we show the temperature. We show only a small area around the
overshoot region so as to highlight the regions where the most change has occurred4. The
bulk of the convection zone remains superadiabatic, however, near the base of the convec-
tion zone, the region becomes slightly subadiabatic. In addition, just below the convection
zone there is a slightly extended mildly subadiabatic region, where the initial steep gradient
between super- and subadiabatic temperature gradients has evolved into a smoother tran-
sition. In this region the temperature gradient (see Figure 3b, region labeled 1) becomes
slightly steeper so that the upward diffusive heat flux can increase to compensate for the
negative convective heat flux in that region (Figure 2b). The depth of the extended mildly
subadiabatic region is roughly 0.05 Hp, if that depth is measured at the point where the
subadiabaticity reaches 1 × 10−5 (which is marked by the arrow in figure 3a). However,
the choice of this depth is arbitrary as it is unclear where the model changes from “mildly
subadiabatic” to “strongly subadiabatic”.
Convective motions are continually pumping heat into the region between labels 1 and
2 in Figure 3b. The timescale for this transfer is much shorter than the diffusive timescale
and therefore, heats up relative to the standard solar model. This mild heating causes a
flatter temperature profile at the region labeled 2 and a steeper profile at region 1. Hence,
region 1 becomes less subadiabatic and region 2 becomes more subadiabatic, which makes
4There is very little change in the deep radiative interior, nor in the bulk of the convection zone.
– 10 –
the change from nearly adiabatic to strongly subadiabatic occur over a shallower depth than
in the standard solar model.
This small difference in temperature between the standard solar model and our evolved,
hydrodynamic model is in the same sense, and at the same radius, as the standard solar
model-helioseismology sound speed discrepancy (Christensen-Dalsgaard 2002). In region 2
the maximum difference in temperature as a fraction of the original temperature (δT/T ) is
∼0.019. This difference has approximately the same amplitude and is in the same direction as
the previous standard solar model-helioseismology discrepancy, before gravitational settling
was included, but is larger than that discrepancy when gravitational settling is included. It is
possible this effect provides an additional explanation for the sound speed anomaly between
the standard solar model and helioseismology.
Because of the extended, mildly subadiabatic region seen in Figure 3, convective motions
can penetrate further into the stable region (Figure 4). In Figure 4 we see that substantial
kinetic energy density, kinetic energy flux and convective heat flux stretch further into the
stable region compared with Figure 2 (before updating the background state). Within the
convection zone, the now non-constant superadiabaticity yields kinetic energy flux and con-
vective heat flux which are not as smooth as they were previously. Key physical features, such
as a positive kinetic energy flux and negative convective heat flux, just below the convection
zone, are still apparent. Subadiabatic overshooting motions in an evolved background state
can extend down to 0.38 Hp (0.687 R)5, still significantly above the base of the tachocline
at ∼ 0.65 R. In summary, we find an extended mildly subadiabatic region down to a depth
of roughly 0.05Hp6, with subadiabatic overshoot extending further to 0.38Hp.
6. Comparison with Previous Results
A review of literature on the subject of penetrative convection yields various predictions
and no consensus. Both numerical and analytic models have been used.Early numerical
simulations in two dimensions (Hurlburt et al. 1994) found that the depth of penetration
depended primarily on the ratio of the sub- to superadiabaticity, S. These simulations found
that for low values of this ratio (1-4), the penetration depth scaled as S−1 indicating an
extended adiabatic region in agreement with analytic results by Zahn (1991) and numerical
5Note that this is as measured by the change in sign of the convective flux and is very similar to thatmeasured by tracer particles (see Section 7)
6Measured at the arrow in figure 3.6a with the stipulation that the definition of mildly subadiabatic issubjective.
– 11 –
results by Singh et al. (1998) and Saikia et al. (2000). For larger values of S (between 5 and
20), the penetration scaled as dpen ∝ S−1/4. Early simulations in three dimensions (Singh,
Roxburgh & Chan 1995) also find evidence for two different scaling laws at low S (1-3) and
moderate S (5-7). However, more recent simulations of turbulent convection in 3D (Brummell
et al. 2002) found only strongly subadiabatic overshoot and no extended adiabatic region,
hence retrieving only the dpen ∝ S−1/4 scaling. The authors argue this discrepancy could be
due to the much smaller filling factor that arises in the 3D simulations or the large velocities
attained in 2D due to flywheel type motion. Our more recent simulations in 2D, using stiffer
values for the stability (closer to solar values) of the underlying radiative regions find no
extended adiabatic region (Rogers & Glatzmaier 2005a). The discrepancies between the
different numerical simulations can be traced to the reference state employed (in particular,
the value of subadiabaticity), the degree of turbulence and the dimensionality of the model.
Analytic models have been in somewhat better agreement. Early models by Shaviv
& Salpeter (1973), van Ballegoojen (1982) and Schmitt, Rosner & Bohn (1984) all found
penetration depths ranging from 20% to 40% Hp7. This appeared to be a robust solution as
they all arrived at these values using vastly different models. The work by Schmitt, Rosner
& Bohn (1984) was the first of the modern analytic solutions which model penetration using
plumes (Zahn 1991, Rieutord & Zahn 1995, Rempel 2004). All of these plume models find
that the penetration depth depends critically on the filling factor, f , that is, the fractional
area occupied by the plumes at the base of the convection zone. Schmitt, Rosner & Bohn
(1984) found that the penetration depth scaled as v 3/2 f 1/2 ; later, Zahn (1991) provided a
derivation of this scaling. He showed that:
d2pen =
3
5HpHχf
ρV 3
Ftotal(11)
demonstrating how the penetration depth depends not only on the filling factor (f ), but also
on the plume exit velocity (V), total heat flux (Ftotal) and thermal diffusivity (through Hχ).
If we compare our model directly to the expression derived by Zahn (1991) for the pen-
etration depth (11), we can isolate any fundamental differences. For instance, both Hp and
ρ are taken from the solar model in our simulations and therefore, can not be criticized as
unrealistic. While our flux is increased by 105 (κm), our velocities are also larger than one
would expect in the solar convection zone, and therefore, the ratio V 3/Ftotal is likely not
unrealistic. Finally, the fundamental complaint generally levied against numerical simula-
tions is their enhanced thermal diffusivity required for numerical stability. At issue is that
7Note that this is the extent of the extended mildly subadiabatic region, not the extent which plumesovershoot (subadiabatic overshoot).
– 12 –
this enhanced diffusivity allows the overshooting plumes to thermalize with the background
thermodynamic state more quickly than they would if the solar thermal diffusivity were
used, hence leading to lower overshoot depths. However, as is seen in (11) the quantity upon
which the penetration depth depends is Hχ, the radiative conductivity scale height. While
our simulation uses an enhanced thermal diffusivity, the radiative conductivity scale height ,
represented by Hχ is taken directly from the solar model and therefore, can not be criticized
as unrealistic. The only factor which may be criticized is the filling factor, which we admit,
may be enhanced because we are only modeling two dimensions.
Using a more sophisticated plume model Rieutord & Zahn (1995) found that when
the effect of the surrounding upflow is included in their plume model the penetration depth
decreases almost linearly with the number of plumes. When the interaction between up-
flows and downflows is neglected, the penetration depth is found to be between 0.2 Hp and
0.4 Hp, as found in previous analytic solutions, but which was inconsistent with helioseis-
mic observations (Christensen-Dalsgaard et al 1995). However, when that interaction is
included the penetration depth depends sensitively on the number of plumes. More plumes
results in more upward momentum loading by upwelling fluid. More recently Rempel (2004)
considered a semi-analytic overshoot model based on plumes. His model also includes the
interactions of plumes with upflows and finds the same behavior of decreasing penetration
depth with increased upflow-downflow interaction (see his figure 7). These results illustrate
that the penetration depth depends, rather strongly, on the upflow-downflow interaction,
which is parametrized in the best analytic models, but self-consistently calculated in numer-
ical simulations. In light of this it seems probable that at least part of the inconsistency
between numerical and analytic models lies in the simplified treatment of upflow-downflow
interaction in analytic models.
In addition to the dependence on mixing between upflows and downflows (parametrized
as α) Rempel finds that the depth of overshoot and the nature of the transition between
convective and radiative zones depends on the ratio of the total flux to filling factor:
φ = Ftotal/(fpbc(pbc/ρbc)0.5) (12)
where f is the filling factor, pbc and ρbc are the pressure and density at the base of the
convection zone, respectively. He finds that the main difference between numerical and
analytic results lies in the values of this ratio; with numerical simulations using values of
φ ∼ 10−2 (because of their increased fluxes) and analytic models employing values around
10−9. The model we present here has φ ∼ 10−3 − 10−4 depending on the filling factor of
the model. Our evolved reference state superadiabaticity (Figure 3a) resembles their model
for φ ∼ 10−4. However, it is difficult to make a direct comparison, given the dependence
– 13 –
of their model on α, although there does appear to be some degree of agreement (slightly
subadiabatic base of the convection zone, slightly extended mildly subadiabatic region).8
To recap, combined numerical and analytic models have found that the penetration
depth depends mainly on: (1) the subadiabaticity of the underlying stable region, (2) the
filling factor at the base of the convection zone and (3) the interaction of upflows and down-
flows. Low subadiabaticity leads to larger penetration depths, while high subadiabaticity
yields small penetration depths. Large filling factors lead to large penetration depths and
vice versa, depending on the number of plumes which occupy that filling factor. For a large
number of plumes the penetration depth decreases, because of enhanced upflow-downflow in-
teraction, while for a low number of plumes the penetration depth increases. Our numerical
simulations accurately represent the subadiabaticity of the radiation zone and the interaction
of upflows and downflows. However, the filling factor depends sensitively on the properties of
the convection (Ra, Re, Pr) and on the geometry and dimensionality of the flow. Resolving
the issue of filling factor remains an important issue, which should be addressed both by
analytic and numerical studies.
7. Tracer Particles
To study the effect of overshooting motions on the mixing of species, we introduce tracer
particles, which are advected by the flow. These particles are introduced after the reference
state has been evolved. We are particularly interested in estimating how deep below the
convection zone the fluid is efficiently mixed.
Five sets of 500 tracer particles were initiated at five different radii, equally distributed
in longitude (Figure 5a). Two sets were initiated in the convection zone (at 0.83 R and 0.79
R, represented in Figure 5 by red and blue points, respectively) and 3 sets were initiated in
the radiative zone (at 0.705 R, 0.690 R and 0.675 R, represented in Figure 5 by purple,
cyan and black, respectively). The yellow line represents the base of the convection zone.
Figure 5b shows the distribution of the particles after two convective turnover times. The
particles initiated within the convection zone (red and blue) are distributed throughout the
convection zone after two convective turnover times. Of the particles initiated in the stable
region many are pulled into the convection zone. The sets initiated at 0.705 R and 0.690
R both have particles within the convection zone in steady state. However, no particles
initiated at 0.675 R have made it into the convection zone on this timescale, although some
8Note that we present the super/subadiabatic temperature gradient, while they show the dlnT/dlnP, sothat the amplitudes are not similar but the profiles of super-/subadiabaticity are similar.
– 14 –
have migrated from their initial positions.
Figure 6 shows the number of particles, of the two sets initiated in the convection zone,
which are pumped into the radiation zone (any radius below 0.718 R) over two convective
turnover times. Both sets converge to having approximately 90 particles (or 18% of the
initial 500 particles) within the radiation zone at any given time. However, most of these
particles do not reach any appreciable depth below the convection zone, existing just beneath
the transition. Figure 7 shows the number of particles that started in the convection zone
and made it into the radiative region as a function of time (left) and radius (right below
the convection zone). The number of particles drops rapidly below the convection zone,
with a nearly linear drop down to 0.705 R and a slightly less rapid drop below that. The
concentrations plotted in Figure 7b are the steady state concentrations; however, the tail
of the distribution (the depth at which no particles are found) depends on the number of
particles initiated, as well as the time simulated, which is very short here.
To ascertain the effect of the number of particles as well as the time simulated (with
particles) on the minimum depth achieved by any particle we ran two additional test cases.
The initial simulation had a total of 1000 tracer particles initiated within the convection
zone, the two test simulations initiated 2000 and 4000 particles within the convection zone,
respectively (half at 0.83 R and half at 0.79 R, as in the original setup). The simulation run
with 2000 and 4000 both particles yielded a minimum depth of 0.684 R, indicating that the
initial 1000 particles were not sufficient to yield a converged result. The two test simulations
were also run longer, a total of four convective turnover times each. The minimum depth of
any particle occurred within the first convective turnover time in all cases, indicating that
the time run (in this small range) does not affect the mixing depth. However, we note that
rare, deep excursions such as those seen in three dimensional box simulations (Brummell et
al. 2002) are not seen here and therefore, a longer integration time would likely have a more
obvious effect in three dimensions.
Of the three sets of particles initiated within the radiative region, only the particles
started at 0.705 R and 0.690 R, are pulled into the convection zone. Figure 8 shows the
number of particles as a function of time, for the three sets of particles initiated within the
radiative region, which make it into the convection zone (any radius larger than 0.718 R).
As can be seen in Figure 9, nearly half of the 500 particles (230, 46%) initiated at 0.705 Rare in the convection zone in steady state. Of the particles initiated at 0.690 R, nearly 40
particles (20 % of those initiated) are within the convection zone in steady state. None of the
particles initiated at 0.675 R make it into the convection zone during these two turnover
times. The maximum radius that any particle initiated at 0.675 R reaches is 0.705 Rduring the time simulated.
– 15 –
The number of particles that are pumped down to the radius 0.690 R (1-2) is signifi-
cantly lower than the number of particles which are dredged up from this same radius (40) in
steady state (compare Figures 7 and 8). That is, there is an asymmetry between the number
of particles dredged up from a particular radius and the number of particles pumped down
to that same radius. This is because downwelling plumes in the convection zone have a small
cross sectional area and are therefore unable to entrain many particles during their descent.
However, once plumes reach the radiation zone their surface area is increased as they are
decelerated. These plumes spread transversely and are turned upward. This “scooping” mo-
tion, allows a greater number of particles to be dredged up than are pumped down. In the
Sun, where the region is completely mixed on timescales much shorter than the evolutionary
timescale, this effect is not important. However, this asymmetry could be very important in
the late stages of stellar evolution of massive stars, when the evolutionary time is closer to
the convective turnover time. This effect could also be crucial in Classical Novae or X-ray
bursts. In these environments the dredge up of underlying heavy material into the accreted
layer affects the energetics and nucleosynthesis of the explosion.
8. Conclusions
We have presented self-consistent, numerical simulations of convective overshoot in a
2D model of the solar equatorial plane. This model employs a realistic thermal reference
state and evolves in response to convection and overshoot. We find that the convective
penetration leads to a thermal profile which is mildly subadiabatic in a shallow region at
the base of the convection zone. Beneath this mildly subadiabatic region the profile drops
precipitously to the extreme subadiabaticity of the radiative interior. Overshooting plumes
cause a mild heating in the overshoot region, leading to a region which has a slightly higher
temperature and a greater subadiabaticity than the standard solar model. The region of
increased temperature is at the location and in the sense of the helioseismology-standard
solar model sound speed discrepancy and may provide an alternative explanation for that
difference.
Passive tracer particles mix only slightly below the convective-radiative interface over
the two turnover times followed. Over longer times, a fully mixed region can be expected
at least down to a radius of 0.684 R. At this radius, the Lithium burning timescale is 6.5
billion years, a factor of 4 to 5 too long. However, only two turnover times were simulated
with tracer particles and its possible that given longer integration time particles initiated in
the convection zone could plunge deeper, thus leading to a shorter burning time. In order
to study this further, longer time integrations are needed. Furthermore, studies varying the
– 16 –
number of particles to determine the minimum radius at which there cease to be particles
should be conducted.
We should bare in mind that these results are 2D and far from the turbulent nature of
the Sun. It is unclear what three dimensional effects would be. While it seems clear that
3D models have smaller filling factors, it is unclear what this affect will be and it depends
not only on the filling factor but the number of plumes accross which that filling factor is
distributed. Furthermore, it is not clear what the effect of meridional circulation or the
full Coriolis force would be on the penetration depth and mixing. Studies on the effect
of Rayleigh number (turbulence level) on the penetration depth indicate that increasing
the Rayleigh number decreases the penetration depth (Brummell et al. 2002; Rogers &
Glatzmaier 2005a). However, it is unclear how these parametrized results come into play
in more realistic models. The jury is still out and awaits more sophisticated, 3D numerical
simulations.
We would like to thank P.Garaud, D.O. Gough, J.C. Dalsgaard, Keith MacGregor,
N. Brummell and R. Rosner for their guidance and thoughtful insight. T.R. would like
to thank the NPSC for a graduate student fellowship. Support has also been provided by
NASA SHP04-0022-00123, NASA NAG5-11220 and DOE DE-FC02-01ER41176. Computing
resources were provided by NAS at NASA Ames and by an NSF MRI grant AST-0079757.
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This preprint was prepared with the AAS LATEX macros v5.0.
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Fig. 1.— Snapshot of the temperature perturbation, representing the full computational
domain. Dark red/white represent cold/hot perturbations with respect to teh background
temperature. The convection region is dominated by descending plumes which overshoot
into the radiative region, finding themselves hotter than their surroundings (white spots at
base of convection zone). Gravity waves are generated by these overshooting plumes.
– 19 –
Fig. 2.— Time and longitudinal average of the kinetic energy density (a), vertical kinetic
energy flux divided by the reference state diffusive heat flux (b) and convective heat flux
divided by the reference state diffusive heat flux(c) before the reference state is updated.
The radius 0.718 R is the interface between the sub- and superadiabatic regions.
– 20 –
Fig. 3.— Reference state thermodynamic model. The solid line represents the initial profile;
dotted line represents evolved (in response to convection and overshoot) model. (a) supera-
diabaticity (positive ∆∇T , as defined in (5)) and subadiabaticity (negative ∆∇T ) in K/cm,
(b) represents the temperature in K. The radius 0.718 marks the transition from convective
to radiative zones.
– 21 –
Fig. 4.— Time and longitudinal average of the kinetic energy density (a), vertical kinetic
energy flux (b) and convective heat flux (c) after the reference state is updated. Again, the
kinetic energy flux and convective heat flux are divided by the original reference state dif-
fusive heat flux. Clearly convective motions penetrate further into the radiative region than
in figure 2. The radius 0.718 was the original interface between the sub- and superadiabatic
regions.
– 22 –
Fig. 5.— Distribution of tracer particles initially (a) and after two convective turnover times
(b) and a zoomed in region after the tracer particles have evolved (c). Red and blue particles
are initiated within the convection zone, while purple, cyan and black are initiated within
the radiative region. The yellow line represents the convective-radiative interface.
– 23 –
Fig. 6.— Number of particles as a function of time in the radiative region that were initiated
within the convection zone. Solid line represents the particles initiated at 0.833 R, while the
dotted line represents those particles initiated at 0.790 R. After two convective turnover
times the two sets of particles converge to having nearly 90 particles (18% of those initiated)
within the radiation zone in steady state.
– 24 –
Fig. 7.— (a) Number of particles as a function of time, initiated in the convection zone, that
make it to the specified radius within the radiation zone. (b) Number of particles starting
in the convection zone that make it into the radiative region (in steady state) as a function
of radius below the convection zone. The solid line in (b) represents a linear decline from
0.715R down to 0.70R and an exponential decline beneath 0.70R.
– 25 –
Fig. 8.— Number of particles as a function of time, initiated within the radiation zone (at
the specified radii, solid line initiated at 0.705 R), dotted line initiated at 0.690 R) which
make it into the convection zone. Note, none of the particles initiated at 0.675 R make it
into the convection zone during these two convective turnover times.